A fuel cell is an electrochemical device that converts chemical energy in the oxidation of fuels (such as hydrogen, methane, butane or even gasoline and diesel) into electrical energy. Fuel cells are simple devices that contain no moving parts, consisting essentially of four functional elements: cathodes, electrolytes, anodes and interconnects. Solid oxide fuel cells (SOFCs) are attractive because of their ability to convert a wide variety of fuels to electrical energy with a high efficiency of up to 70% in pressurized systems as compared to engines and modern thermal power plants that typically show a maximum of 40% efficiency. In applications designed to capture the SOFC's waste heat for co-generation, the overall efficiency can top 80 percent. SOFC technology has the distinct advantage over competing fuel cell technologies (e.g. molten carbonate, polymer electrolyte, phosphoric acid and alkali) because of its ability to use fuels other than hydrogen and their relative insensitivity to CO, which act as poisons to other fuel cell types.
The general design is that of two porous electrodes separated by a ceramic electrolyte. The oxygen source, typically air, contacts the cathode to form oxygen ions upon reduction by electrons at the cathode/electrolyte interface. The oxygen ions diffuse through the electrolyte material to the anode where the oxygen ions encounter the fuel at the anode/electrolyte interface forming, water, carbon dioxide (with hydrocarbon fuels), heat, and electrons. The electrons transport from the anode through an external circuit to the cathode.
Although SOFCs are, in concept, simple, the identification of efficient materials for the components remains an enormous challenge. These materials must have the electrical properties required, yet be chemically and structurally stable. State of the art SOFCs operate at temperatures of about 1000° C. to achieve sufficiently high current densities and power. The reactivity of the components with each other and/or the oxygen and/or the fuel and the interdiffusion between components presents a challenge at the high temperatures. The thermal expansion coefficients of the materials must be sufficiently matched to minimize thermal stresses that can lead to cracking and mechanical failure. The air side of the cell must operate in an oxidizing atmosphere and the fuel side must operate in a reducing atmosphere.
One of the most common electrolyte materials for fuel cells is yttria-stabilized zirconia (YSZ). Yttria serves the dual purpose of stabilizing zirconia in the cubic structure at low temperatures and providing oxygen vacancies. As an alternative to YSZ, doped cerium oxide and doped bismuth oxide have shown some promise, however, neither are sufficient to perform as needed. Bismuth oxide-based electrolytes have high oxygen ion conductivities sufficient for low temperature operations (less than 800° C.) but require high PO2 levels for sufficient thermodynamic stability. Low PO2 at the anode promotes bismuth oxide decomposition, and results in failure of the SOFC. Cerium oxide based electrolytes have the advantage of high ionic conductivity in air and can operate effectively at low temperatures (under 700° C.). However, these electrolytes are susceptible to reduction of Ce+4 to Ce+3 on the anode, leading to electronic conductivity and a leakage current between the anode and cathode. A temperature below 700° C. significantly broadens the choice of materials for the cathodes, anodes, and interconnects, which allows for the use of much less expensive and more readily available materials than those used currently for SOFCs.
In addition to the need for a superior electrolyte, the anode and cathode need improvements to form excellent SOFCs. Improvements not only involve identifying superior materials, but also identifying improvement of the triple phase boundary between the electrode, electrolyte, and oxygen or fuel. Hence, viable low temperature SOFC requires identification of a system, materials, structure and fabrication techniques that maximizes efficiency at the minimum temperature.